Regeneration of exhausted silver bleaching solutions



and by the loss of bromide ions.

Patented Sept. 23, 1952 REGENERATION OF EXHAUSTED SILVER BLEACHING SOLUTIONS Robert H. Zappert, Binghamton, N. Y., assignor to General Aniline & Film Corporation, New J York, N. Y., a corporation of Delaware No Drawing.

10 Claims.

This invention relates to a photographic process and more particularly ,0 the process of regenerating exhausted silver bleaching solutions.

In the processing of color film it is customary to convert the silver, formed by development, into a hypo-soluble silver salt, such as silver bromide which is subsequently removed by fixing. A similar procedure is involved in removing negative silver in processing black and white reversal film. The most commonly employed silver bleaching solution consists essentially of an aqueous solution of an alkali ferricyanide and an alkali bromide, and if desired, small amounts of a buffer or a bleaching accelerator, such as the phenoxyethanol described in United States Patent 2,419,900.

The bleach solutions, although reasonably stable during storage, lose their efiiciency as soon as they have been used for the treatment of relatively small areas of film. Subsequent bleaching with the used solution not only requires considerably more time but the removal of silver is incomplete, causing dulling of the highlights together with unsightly stain. Replenishments of the customary procedures, such asby the addition of ferricyanide and potassium bromide,"

which are used up in the bleaching, reaction, does not improve the performance of an exhausted bleach solution to any appreciable extent. The analysis of such a solution intended for discard usually indicates that as'much as 90 to 95% of the alkali ferricyanide originally added is still present. It becomes manifest that a procedure utilizing only a small fraction of an active ingredient is not only wasteful but.

economically unsound, particularly where as here, thhe activeingredient, the ferricyanide, is relatively expensive. It is, therefore, an object of. the present invention to provide a simple and economical method of, regenerating an exhausted ferricyanide bleach solution. l

Other objects and advantages will appear hereinafter. i

According to the present invention, exhausted bleach solutions comprising an alkali ferricy- Application December 30, 1949, Serial No. 136,145

anide and an alkali bromide are regenerated or rejuvenated by the action of'a reagent or mixture of reagents capable of releasing elementary bromine in aqueous solutions. recognized that the slowing up of thebleaching rate of an exhausted silver bleach solution is caused by the formation of ferrocyanide ions The action of bromine on an exhausted bleach solution appears It has been to perform a dual function (1) by oxidizing the ferrocyanide ions to ferricyanide ions, and (2) by supplying additional bromide ions. Bromine, itself or its solution in water commonly known as bromine water, performs a similar function but tends to destroy some of theferricyanide. Moreover, bromine gives off highly objectionable toxic vapors and must be handled with extreme care in order to avoid serious burns. In addition, if accidentally spilled, it may seriously affect not only the film but also tanks, spools, and similar equipment utilized in the film processing.

I have found that reagents or mixtures of reagents containing bromine as a hypobromide or a bromate-bromide mixture and which release bromine after addition to the exhausted bleach solution may be employed to overcome the foregoing disadvantages. Reagents of this type are the alkali metal hypobromides, i. e., sodium, potassium or lithium hypobromide, a mixture of an alkali metal bromate and hydrobromic acid in the molar ratio of 1:6, and a mixture of an alkali metal bromate and alkali metal bromide in a molar ratio of 1:5, and containing in addition approximately 6 to 7 mols of either sulfamic acid or an anhydrous alkali bisulfate. The anhydrous alkali biculfate may be replaced by 3 to approximately 3.5 mols of sulfuric acid diluted with a sufiicient amount of water to make approximately a 20% solution. 4

The amount of the alkali metal hypobromide or mixture of alkali metal bromate and hydrobromic acid, and the like, is such as will release one molar equivalent of bromine for each molar equivalent of ferrocyanide present in the exhausted bleach. The actual, or approximate amount can, therefore, be determined stoichiometrically after the determination of the ferrocyanide content by colorimetric, potentiometric, or other suitable titration means. Amounts of the reagent or molar equivalent mixture thereof smaller than the molar equivalent of ferrocyanide present in the exhausted bleach bath give incomplete regeneration, whereas any appreciable excess tends todestroy some of the ferricyanide. A small excess can, however, be tolerated in most instances without impairing the eificiency of the regenerated bleach solution.

As is well known all photographic bleach solutions contain an alkali metal ferricyanide, such as lithium, potassium or sodium ferricyanide as an oxidizing agent dissolved in a suitable solvent, such as, for example, water with or without thepresence of other adjuva'nts. A typical bleach solution consists of about 100 grams of an oxidizing agent per liter of solution and may contain one or more of the following ingredients:

Alkali metal bromide, such as lithium, potassium I and sodium bromide grams (converting agent) Dialkali metal hosphate 40 grams (buffering agent) Alkali metal 1), sulfate or organic acid, such as citric acid 5 grams (buffering salt) Alkali metal tetraphosphate grams (COIIOSIOD preventative) Water to make 1 liter (solvent) By an oxidizing agent, I mean any alkali metal ferricyamde that will oxidize axmetallic silver image to form a silver salt. "Most commonly used oxidizing agents are alkali metal -'ferricy-' anide, such as lithium, potassium,.and sodium ferricyanides or mixtures of these salts.

By'a converting agent, I mean a compound such as an alkali metal bromide which by reason of a difference in solubility will replace the silver ferricyanide formed in. the bleaching reaction to form silverbromide and .thus impel the le ch. Converting a nts a sometimes called halogenizingagentsreaction to completion.

By buffering agents, I means. substance that is added to bring theferricyanide bleach solution .to a specified pH and to maintain this-pH even though small amounts-20f acids oraa ka may be carried intozthle bleach of the him. The

pH of the solutions,used-in-bleaching films are maintainedbetween 3 and .9. although a-pH between 5and 7 is generally preferred. Y 1

By solvent, I refer to such liquid in which above. mater als are dissolved and Which is usuallywater. Y By'corrosion preventatiyestl refer to those compounds which retard :thea-corrosion of the ferricyanide bleach. on developing. tanks, trays,

and the like. Large quantities (20pto 80grams per liter) of alkali metal phosphate, such as disodium phosphate sodium tetraphosphate, and the like have been found to prevent corrosion effectively.

The following. examples describe in detail, methods for accomplishing the above objects; but

it is understood that they are inserted merely for the purpose of illustration and are notto be construed as limiting the scope of the invention.

' Exarriplel 7 7 I v A photographic bleach 'solutionhaving' the fol-- lowingformula:

" 'Gram's Potassium' ferricyanide i 100- Potassium bromide 15 Dibasic sodium phosphate 40 Sodium bisulfate 25 Water to make 1 liter.

was used for the bleachingof 45 feet of 35 mm. color developed reversal film.} I g I i Photographic tests on sensitometric strips J showed that thebleaching rate. of. the used or exhausted bleach solution, dropped below the practical limit and that even extended bleaching" of a small length of color developed 35 mm. re-.

-versal film failed to remove the residual silver stain. V

The ferrocyanide ion concentration of the exhausted bleach solutionrwas determined, according to the method of Brunner. Means and-Zappert, described inlthe Journal of the Society'of Motion Picture Engineers, vol. 53, page 3O, and

found to be 8.5 rams per liter.

A solution of sodium hypobromid prepared by calculated from the results .of theanalysis as the chemical equivalent of the ferrocyanide present, was added to the'exhausted bleach solution. The

3 pH of the mixture was adjusted to approximately 8.5. After the solution had been standing for a short while, the pH was readjusted to 6.5 by the addition of dilute sulfuric acid.

Photographic tests showed that the bleaching j actionof the rejuvenated bleach was equal to that of .a fresh solution.

Example II One 'liter of a fresh bleach solution prepared as in Example I was used for the bleaching of 50 feet of color developed 35 mm. film. After bleaching, the solution showed signs .of exhaustion andnpon colorimetric analysis indicated .a

ferrocyanide ion v.concentrati'on oflu grams per,

liter. 1 V

In one liter of the exhausted bleach solution, 1.31 grams molar equivalent) of potassium bromate were first dissolved and then 19 mls.

(1 molar equivalent) of 20% hydrobromic acid,

added with stirring.- After standing for less than 10 minutes, the solution was completely rejuve-v nated and showed excellent-bleachingproperties.

The pH of the regenerated solution did not require adjustment. s I

Example III One liter of an exhausted bleach solution, of he same composition as in; Example I, was analyzed by iodometric titrationand-found to contain 10. grams of .ferrooyanideion per liter.

The bleach solution was regenerated by adding,

successively, a solidmixture of 1.31 grams /6 molar equivalent based on'the amount of ferro-' cyanide in the bleach) of potassium bromate and 4.68 grams molar equivalent) of potassium.

bromide, followed by the addition of :5.66 grams (1 molar equivalent) of anhydrous sodium. bisulfate. Within a few minutes, the solution was completely rejuvenated and did not require any further adjustment of its pH; r.

Example -I-V Example III was repeated with the exception that 5.66 grams of anhydrous sodium bisulfate were replaced by 9. 9 mls. of a 20% aqueous solution of sulfuric acid. The exhausted bleach solution was completely rejuvenated and did not re-- quire any further adjustment of its pH.

In the commercial processing of motion'ipicture color film, the concentration of the various constituents of the bleach solution decreases gradually. Some water is brought into the bleaching tank by the Wetted film and-small quantities of the ferricyanide salts are carried away'b'yfthe bleached film. Thi dilution in most casesdoes not materially afiect the. bleaching ratej 'If'desired, the solution can, however, be kept at full strength byadding small amounts of the same constituents or chemicals originally used for the preparation of the bleach solution.

As many apparently widely different embodi-' I ments of this invention may bemadewithout departing from the spirit and scope'thereof, it is o e nd rst od that I do not wish to limit mydefined by the appended claims.

self to the specific embodiments thereof except as:

I claim:

1. In the photographic process of bleaching a metallic silver image to silver halide with a bleach solution comprising an'alkali metal bromide and ferricyanide whereby the silver in the silver image is oxidized to silver ions which combine with the halide ions of the solution and form a silver halide in the emulsion and a relatively small amount of the ferricyanide is reduced to ferrocyanide, and then regenerating the spent bleaching solution by adding a regenerating agent, the improvement wherein said regenerating agent is a material selected from the group consisting of an alkali metal hypobromide, a mixture of alkali metal bromate and hydrobromlc acid in the molar ratio of 1 to 6, and a mixture comprising an alkali metal bromate and an alkali metal bromide in the molar ratio of 1 to 5, the amount of said material being such as that which will release one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleach solution.

2. A process according to claim 1, wherein there is added an amount of an alkali metal hypobromide in a quantity sufficient to release one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleach solution.

3. A process according to claim 1, wherein there is added an amount of a mixture of an alkali metal bromate and hydrobromic acid in the molar ratio of 1 to 6 in a quantity sufiicient to release one molar equivalent of bromine for each molar equivalent of ierrocyanide ion present in the exhausted bleach solution.

4. A process according to claim 1, wherein there is added an amount of a mixture comprising an alkali metal bromate and alkali metal bromide in the molar ratio of 1 to 5 in a quantity sufiicient to release one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleach solution.

5. A process according to claim 1, wherein there .is added an amount of sodium hypobromide in a quantity suflicient to release one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleach solution.

6. A process according to claim 1, wherein there is added an amount of a mixture of potassium bromide and hydrobromic acid in the molar ratio of 1 to 6 in a quantity sufficient to release one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleach solution.

7. In the photographic process of bleaching a metallic silver image to silver halide with a bleach solution comprising an alkali metal bromide and an alkali ferricyanide whereby the silver image is oxidized to silver ions which combine with the halide ions of the solution and form a silver halide in the emulsion and a relatively small amount of the ferricyanide is reduced to ferrocyanide, and then regenerating the spent agent, the improvement wherein said regenerating agent is a mixture of potassium bromate and potassium bromide in the molar ratio of 1 to 5 containing 6 to 7 mols of an alkali metal bisulfate in a quantity sufficient to release one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleach solution.

8. In the hotographic process of bleaching a metallic silver image to silver halide with a bleach solution comprising an alkali metal bromide and an alkali ferricyanide whereby the silver in the silver image is oxidized to silver ions which combine with the halide ions of the solution and form a silver halide in the emulsion and a relatively small amount of the ferricyanide is reduced to ferrocyanide, and then regenerating the spent bleaching solution by adding a regenerating agent, the improvement wherein said regenerating agent is a mixture of potassium bromate and potassium bromide in the molar ratio of 1 to 5 containing 6 to 7 mols of suliamic acid in a quantity suihcient to release one molar equivalent of bromine for each molar equivalent of ferrocyanlde ion present in the exhausted bleach solution.

9. In the photographic process of bleaching a metallic silver image to silver halide with a bleach solution comprising an alkali ferricyanide and an alkali metal bromide whereby the silver in the silver image is oxidized to silver ions which combine with the halide ions of the solution and form a silver halide in the emulsion and a relatively small amount of the ferricyanide is reduced to ferrocyanide, and then regenerating the spent bleaching solution by adding a regenerating agent, the improvement wherein said regenerating agent is a mixture consisting of an alkali metal bromate and an alkali metal bromide in the molar ratio of 1:5 and which said mixture of the alkali metal bromate and alkali metal bromide contains 3 to 3.5 mols of sulfuric acid dissolved in a sufiicient amount of water to make a 20% aqueous solution. I

10. The process according to claim 7 wherein the alkali metal bisulfate is sodium bisulfate.

ROBERT H. ZAPPERT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number OTHER REFERENCES Procedure for Rejuvenating Color Film Bleach Solutions, published by Ansco, received in Div.

67 on October 27, 1947, 11 pages; page '1 cited.

McPherson, Henderson, Mack and Fernelius, Chemistry Ginn & C0., N. Y., 1940, pp. 313-314. 

1. IN THE PHOTOGRAPHIC PROCESS OF BLEACHING A METALLIC SILVER IMAGE TO SILVER HALIDE WITH A BLEACH SOLUTION COMPRISING AN ALKALI METAL BROMIDE AND FERRICYANIDE WHEREBY THE SILVER IN THE SILVER IMAGE IS OXIDIZED TO SILVER IONS WHICH COMBINE WITH THE HALIDE IN THE EMULSION AND A RELATIVELY SMALL HALIDE IN THE EMULSION AND A RELATIVELY SMALL AMOUNT OF THE FERRICYANIDE IS REDUCES TO FERROCYANIDE, AND THEN REGENERATING THE SPENT BLEACHING SOLUTION BY ADDING A REGENERATING AGENT, THE IMPROVEMENT WHEREIN SAID REGENERATING AGENT IS A MATERIAL SELECTED FROM THE GROUP CONSISTING OF AN ALKALI METAL HYPOBRIMIDE, A MIXTURE OF ALKALI METAL BROMATE AND HYDROBROMIC ACID IN THE MOLAR RATIO OF 1 TO 6, AND A MIXTURE COMPRISING AN ALKALI METAL BROMATE AND AN ALKALI METAL BROMIDE IN THE MOLAR RATIO OF 1 TO 5, THE AMOUNT OF SAID MATERIAL BEING SUCH AS THAT WHICH WILL RELEASE ONE MOLAR EQUIVAENT OF BROMINE FOR EACH MOLAR EQUIVALENT OF FERROCYANIDE ION PRESENT IN THE EXHAUSTED BLEACH SOLUTION. 